Quick response oxygen analyser

ABSTRACT

A quick response paramagnetic oxygen analyzer wherein analysis and reference gas are passed through a gap space in a periodically excited ferromagnetic circuit affording a homogeneous or uniform zone and an inhomogeneous or nonuniform zone of the magnetic field. Intermixing and diffusion of the gases can occur only in homogeneous zone and in an exit conduit. A pressure difference detector for the analysis and reference gases at their entrance into the inhomogeneous zone measures relative susceptibilities and hence oxygen content of the two gases.

United States Patent [72] Inventor Heinz Hummel I 2,903,883 9/1959 Konigstein-Johanniswald, Germany 3,287,959 1 H1966 [21] App]. No. 756,655 3,049,665 8/1962 [22] Filed Aug. 30, 1968 3,240,051 3/1966 Lemfant [45] Patented June 15, I971 FOREIGN PATENTS [731 Assign 1" Akfimgesel'scha 1,149,187 5/1963 Germany 73/27 5 Frankfurt am Main, Germany [32] P i i A 31, 1967 Primary Examiner-Richard C. Queisser [33] Germany Assistant Examiner-Ellis J. Koch [31] P 16 48 924.0 Attorney-Franklin R. Jenkins [54] QUICK RESPONSE OXYGEN ANALYSER 3 Claims, 5 Drawing Figs.

[52] U.S. Cl 73/23, S R A quick response paramagnetic Oxygen analyzer 73/27-5 wherein analysis and reference gas are passed through a gap [51] Int. Cl C0111 7/00, Space in a periodically excited ferromagnetic circuit affording 27/00 a homogeneous 0r uniform zone and an inhomogeneous or [50] Field Of Search 73/23, 27.5, nonuniform Zone f h magnetic field. Intel-mixing and diffu 26? 324/36 sion of the gases can occur only in homogeneous zone and in an exit conduit. A pressure difference detector for the analysis [56] Reerences cited and reference gases at their entrance into the inhomogeneous UNITED STATES PATENTS zone measures relative susceptibilities and hence oxygen con- 2,696,73l 12/1954 Luft 73/23 tent of the two gases.

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SHEET 2 BF 3 PATENTED JUN] 51% SHEET 3 [IF 3 QUICK RESPONSE OXYGEN ANALYSER BACKGROUND OF THE INVENTION 1. Field of the Invention.

A paramagnetic gas analyzer making use of pressure changes on sample and reference gases in the inhomogeneous zone of a magnetic field.

2. Description of the Prior Art.

There have been numerous processes and devices for the measuring of magnetic susceptibility or the oxygen gas content in mixtures based on the relatively high paramagnetic susceptibility of oxygen. The thermomagnetic methods, as well as those dependent on diffusion about a heated filament, relying on sluggishness of molecules have a response time of about 0.1 second. Also those analyzers working on the Pauling principle cannot match this quick response because of the large volume and limited flow rates.

German patent 840,614 describes a device but it cannot be quick acting since the pressure-measuring instruments for the required reference pressure of from 0.001 to 0.l mm. water column are too sluggish.

In the German Auslegesschrift 1,149,187 a quick analyzer is described, but it requires a frequency above 25 Hz. For magnetic circuits operating at such frequencies, especially including high permeability ferromagnetics, only core trays may be used and this usage introduces a considerable magnetomechanical disturbance which is also dependent on carrier gas. lnstruments utilizing thermomagnetic effects to cool a heated filament are also influenced by carrier gas and are quite delicate of construction and require much adjustment. Analyzers operating on the Pauling principle are affected by gas through put rates, and carrier gas, to mention only a few objections. Moreover almost all the above-mentioned instruments require unusual types of pole pieces, particularly those making use of an inhomogeneous or nonuniform magnetic field.

In Swiss patent 280,228 there is a combination using a rotating field and which includes small inlet tubes for analysis and reference gases and which has a common outlet tube. However this combination has the drawback that the decisive inhomogeneous zone for the modulating effect, that is to say the line of separation of the two half circuits of iron or nonferromagnetic material, shifts over the measuring tubes. Since the concentration profile in the mixing zone is not predetermined, and since variation on either side has a decided influence on the measuring effect, there is consequently a large source of error greatly depending on the density and viscosity of the gases, as well as their velocity of flow. Besides the mixing zone is spread out by the rotating magnetic field.

The present invention has the following advantages:

1. Since in the case ofa sudden variation in concentration of analysis gas only a few millimeters ofgas volume present in the inhomogeneous zone must be renewed by the gas of altered content, and this can be done in 0.l sec. when the flow rate is as low as one liter per hour.

2. The flow rate has no influence on the resulting measurement over a wide range.

3. The presence of a carrier gas in the analysis gas has practically no influence on the resulting measurement in as much as the diamagnetic susceptibility does not play any part for oxygen concentration less than 0.5 percent.

4. Mechanical or thermal properties of the analysis gas such as density, specific heat, viscosity and diffusion constant have no influence on the measuring results. This enables specific calibration to be computed from a datum calibration.

5. The requirements for mechanical precision finishing are very few.

SUMMARY OF THE INVENTION The invention consists of an apparatus comprising a closed ferromagnetic, periodically excited magnetic circuit having a gap into which analysis gas and a comparison or reference gas of known magnetic susceptibility are led by, respectively, two small conduits, the two gases leaving through a common small exit conduit. A detector for measuring alternating pressure differences in the two conduits is provided. A parallel gap is employed with a practically homogeneous or uniform field, the admission of the analysis and reference gases into the homogeneous field are effected in such a manner inside that an intermixing of the two gases and an alteration of the composition of the gases by mutual diffusion can occur only in the homogeneous zone and in the exit passageway so that in the inhomogeneous zone of the magnetic field the analysis gas inlet duct and the reference gas inlet duct are always filled with their respective gases.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows one form of the invention.

FIG. 2 shows a cross section of a cell for insertion between poles of a magnet, the section being taken along the line 2-2 of FIG. 2a looking in the direction ofthe arrows of said line.

FIGS. 2a and 2b show the cell in position and FIG. 3 shows another form of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An alternating magnetic field is set up in a magnet 1 by means of an exciter coil 2 fed by pulsating DC derived from 50 Hz. alternating current line by means ofa rectifier 3. The magnet has a parallel transverse slot with a gap space 4 and the gap space terminal ends or seals 5 and 6. Analysis and reference gas flow through small tubes 7 and 8 respectively and are drawn from thence through valves 9 and 10 and the entrance ducts 11 and 12 into the space of the slot. The gas is then drawn from the space 4 commonly through exit tube 13, a valve 14, a surge chamber 15 and a valve 16, by means of a suction pump I7. The U-tube 18 indicates suction pressure for the adjustment of through-put and for control purposes. The inlet ducts l1 and 12 are connected by conduits 19 and 20 to an alternating or changing pressure pickup 21 which is a capacitor microphone made up of a membrane and a perforated plate or plates. The microphone and a source of DC 22 are serially connected with a working resistor 23 of about a million ohms. The working resistor shunts the input of an AC voltage amplifier 24 at proper phase rectification. The rectifier receives its control signals from a control coil 25 on the magnet. The control signal is induced in the control coil by the changing or alternating flux of the magnet. The recorder 26 registers the output DC signal of the amplifier.

If the analysis and reference or comparison gases are supplied at super atmospheric pressure the pump 17 and U-tube 18 may be omitted.

The mode of operation is such that a special homogeneous magnetic field pervades the space 4 fluctuating periodically between zero and H in strength, and at the frequency of 50 Hz. Due to the position of the inlet or outlet positions of tubes l1, l2 and 13, the homogeneous field is disturbed. For the remaining field space it is assumed that the homogeneous field pattern is practically undisturbed. The valves 9, l0 and 14 protect the inner measuring system composed of the gap space 4, with the pickup, besides the conduits ll, l2, l3, l9 and 20 against pressure disturbances and pressure fluctuations from the gas entrance and exit and at the same time and assure a continuous predetermined flow. The valves are in effect resistors to gas flow so that ripples in pressure in the main feed lines and exit line will not be transmitted to thegap space and interfere with pressure fluctions therein due to the fluctuating magnetic field. The entrance gas conduits 11 and 12 are filled on account of the continuously supplied fresh gas until the extent of the homogeneous field is always filled inside by analysis or reference gas. The susceptibilities of the gases in both gas streams depend proportionately on the oxygen contents and are designated K, and k,.. l(,, being that of the gas analyzed for and K being that of the comparison gas. It can be demonstrated that the difference of alternating pressure is proportional to the susceptibility difference K,,,K,.. Only the conduit pieces 11 and 12 play a part on the transition point from the field free space longitudinal channel into the inhomogeneous or nonuniform field space, since inside the homogeneous or uniform field very slight force action is exerted on the oxygen. Of special importance is the discovery that the field pattern in the nonuniform zone has no influence on the quantity of the measuring effect. This fact makes the invention very superior in the art of gas analysis.

Details of another form of the measuring cell providing the space 4 are shown in FIGS. 2, 2a and 2b. The cell C is made up of two nonmagnetic plates 34, 35 of about 5 mm. in thickness between which is soldered, clamped, screwed or pressed together a brass or steel platelike frame of about 0.25 to 0.50 mm. in thickness which surrounds the oval-shaped gap space 4 proper. The cell thus has upper and lower parallel exterior faces and is shoved in between the parallel end faces of the magnet l. L-shaped passageways 31 and 32 in the plate 35 lead analysis and reference gas from their respective conduits 11 and 12 into the space 4 generally at opposite end portions thereof, while an inverted similar passageway 33 at the midportion, provides for the removal of both gases.

The magnet l is built of iron lamanae highly permeable at frequencies ofthe order of 5-60 Hz.

The use of a cell as described is necessary to avoid having a magnetostrictive alternation pressure effect leading to a zero effect. Perhaps there is here only a mechanical effect as the result of vibration of individual magnet lamanae.

The use of this present cell has avoided the tied in zero effect with the lamanae core construction effectively. Use of highly permeable lamanae gives a further advantage in enabling a very strong magnet field to be produced in the space 4 with a comparatively small power loss. Moreover, it is possible to attain adequate measuring effect with strip cores of only an area of X15 mm. cross section.

In another form of the invention the measuring cell is not formed directly on the magnet but mounted out of actual contact from the magnet in the air gap, the magnet and cell are thus independently mounted on a mounting block. This permits a further reduction of the magnetomechanical disturbing effect but at the same time increases the reluctance of the magnetic circuit and hence there must be a larger number of ampere turns to produce the same field strength.

Since, as explained, it makes no difference in which manner the nonuniform zone is traversed, it is here possible that the analysis or comparison gas be led in from the side of the cell, for example through the chamber terminals 5 and 6 as in FIG. 1, always a short small entrance tube, or better yet, a flat walled slot.

A practical application of the analyzer is the determination of the oxygen content in human exhalations where a range of from 20.5 percent to 12.5 percent oxygen is sufficient. Ambient air can be used as the reference gas. The recording delay or response time is less than 0.1 sec. and enables the registration of oxygen content during hyperventilation or a pant as short as one second in medical diagnosis.

The analyzer may also be used to determine oxygen contents in rapidly occurring partial combustion oxidation in chemical reactors and in exhaust gas from internal combustion engines.

The use of the analyzer in gas chromotographic work for quick analysis in combination with columns of capillary dimensions is highly advantageous. With oxygen as the carrier gas and concentration of the same order as those used with heated filaments, the smallness of cell volume and rapid response enable the analysis of samples which was hitherto very difficult.

The present analyzer was tested at frequencies from 3 Hz. to 100 Hz. At 3 Hz. a massive pot magnet arrangement was satisfactory. Use at frequencies higher than 100 Hz. should be ofinterest largely in respect to reducing amplifier input.

Since it is impractical in many technical applications to prepare and have a supply of proper comparison gas, a modification of the invention enables a comparison gas to be produced from a portion of the analysis gas. In this form of the invention a portion of the analysis gas is brought to a higher temperature by about C., for example before entrance into the gap space and directly before the beginning of the nonuniform field zone. By use of appropriate dimensions of the gas ducts and a heating system this portion, as comparison gas can be brought to a high temperature T,. in the range of the nonuniform zone of the field and be substantially constant and be magnetically independent of the presence of carrier gas. This requirement was met in an apparatus for an industrial oxygen analyzer where the range was 0-5 percent oxygen with an allowable error of 0.1 percent oxygen when the temperature of the gas did not decrease more than about 3 upon passage through the nonuniform zone.

In comparison with the thermomagnetic methods the modification described next above has the following advantages.

(1 The effective temperature field lies outside the effective magnetic field. There is no stipulated relationship to keep between the magnetic field and the temperature field. By the sifting of the entire temperature field outside the magnetic circuit a sufficiently large heating system is possible. Thus a reference gas temperature T, independent of the composition and flow rate ofthe gas as well as the inclination of the instrument can be obtained.

(2) There is here a measuring effect proportional to the difference in susceptibilities K,,,K,- at T (say, 40 C. or 313 K.) and T,-(at C. and 413 K.). The diamagnetism of the carrier gas is proportional to density or l/Twhile the paramagnetism of oxygen varies inversely with the square of the absolute temperature l/T. For an oxygen mixture having diamagnetic carrier gas there is the relationship 1 im-r zl wherein C is the concentration of oxygen to be measured. The measuring effect is therefore independent of density, viscosity, and specific heat of the carrier gas. With sufficiently great heat exchange to heat the incoming gas there is no effect due to the presence of carrier gas. This is not true of any of the conventional oxygen analyzers. A minimizing of the diamagnetic disturbing effect by a factor 2 is as good as can be obtained in most merino-magnetic instruments yet by the present invention a favorable factor is obtained because the varying carrier gas disturbing effects can be essentially avoided.

FIG. 3 shows a form of the invention for use with the heated comparison gas of the same active composition as the analysis gas. The combination comprises a magnet 41 providing an alternating field, a gap chamber 42 formed as a cross channel (about 0.5 X6 mm.) provided with chamber seals 43 and 44 of insulating material, each having a rectangular entrance opening (for example 0.5 X6 mm). Sample gas to be analyzed enters a conduit network at a T-connector 51 and is split into two streams carried by right and left conduits 52 and 53 under the control of valves 54 and 55 and thence by extensions 48 and 49 to opposite sides of a differential pressure detector 50. Connection of the channel 42 to opposite conduit leads 52 and 53 is by right and left conduit pieces 45 and 46 tapping the respective leads, the conduit piece 45 being heated by a heater 47 while the piece 46 remains unheated. It is contemplated that pieces 45 and 46 remain at constant different temperatures. Exit gas leaves the channel 42 by conducting means 56 under the control of valve 57 and tapping the channel at right angles substantially at the mid point of the channel. The detector 50 feeds a circuit similar to that for detector 21 in FIG. I.

The invention according to FIG. 3 can also be used in such a way that the analysis gas passes in only through the valve 54 and out past the valve 55, with the connection between these two valves, as well as conduit 56 and valve 57, being omitted. In this mode of operation the heated analysis gas entering into the gap space leaves by tube 45, upon passing through the channel 44 situated in the inhomogeneous zone, leaving the latter at the temperature existing before heating.

In the Swiss patent No. 280,228 in FIGS. 1 and 2 an arrangement with an alternating magnetic feed is fully described in which the analysis is heated locally and so two gas zones are produced of varying magnetic susceptibility. However there the heating is in the nonuniform zone, where the analysis gas flows out of the magnetic field. In that method only a portion of the difference in susceptibilities is reflective as a measure of oxygen content. Besides, the difference effect is highly dependent on flow rate, gas properties such as density, viscosity, specific heat, etc. as well as heat conductivity. Care must be taken that analysis and comparison gas present in the range of the homogeneous field be in complete pressure equilibrium.

These requirements can be met by taking the following measures.

1. By a narrowing of cross section of the entrance ducts in the range of the inhomogeneous zone of the field, on account of the accompanying increase in flow rate, the entrance of one constituent gas (for example the analysis gas) prevents the penetration in the other (the reference) gas.

2. The distance between the points of entry of the analysis and reference gas from each other must be sufficiently great. The amount of this distance depends on the geometry of the slot space, the flow rate of the gases, the frequency, the difference in paramagnetic susceptibilities and on field strength. This distance can usually be best determined by experiment but in cases where the geometry of the slot or gap space is simple the distance may be calculated.

3. The common exit means for both gases may be a small bore or channel so positioned and located that the two gases combine over as wide a cross section as is possible in the homogeneous field space.

For the measuring effect it is immaterial whether the entrance of the analysis gas and reference gas is through transverse or longitudinal channels or transverse or longitudinal slits. It is also immaterial, where the tappings to the differential pressure detector are provided if it is only surely afforded that these tappings are provided at locations ahead of the entrance of the respective gases into the inhomogeneous zone. The impulse takeoff can however be provided by additional passageways outer from the gap space directly. In use of the analyzer generally it is almost necessary that the entrance passageways for pressure difference detector, upon variation in concentration of the analysis gas, be first filled with the newly changed gas by diffusive motion in the inhomogeneous zone. In practice this causes some delay in response.

It is also possible to mount both feed ducts for the analysis and reference gases respectively directly beside each other and to separate the two by only means of a common partition wall or web. The partition wall dividing the duct passageways may project into the homogeneous field zone but it must not obstruct the space thereof to the extent of interferring with a complete equalization of pressure in the homogeneous zone.

In a form of the invention according to FIG. 1, zero oxygen difference between analysis and reference gas exhibited a null effect" and this effect varied with the carrier gas content. This null effect was produced by a magnetomechanical alternating pressure which, on account of the asymmetrical construction of the pickup and the asymmetrical infeed of the conduits, caused an alternating pressure signal in the pickup. This action was overcome by two measures. The first con cemed the slot-space ends or seal and the second was the developing of the slot space as a mechanically stable cell as in FIG. 2. The poles of the magnet attract each other with a force proportional to the square of the field strength. Due to this force the slot space seal formed as a housing was compressed. This compression led to a limitation in volume of the slot space, and as a result from that, to an alternating pressure in the slot space. By enlarging the frames cross sectional area by a factor of 5, this effect could be practically eliminated, since the peak pressure was reduced by a factor of 5, and at the same time the compressed" volume is reduced by a like factor.

A further means for reducing this asymmetry is to flush the conduits to the differential pressure pickup with uniform gas in the region of the field-free space outside the magnetic circuit.

This may be done in a self-evident way so that the fluid gas does not enter the gap space, as by a sealing off of the conduits 19 and 20 near their connection to pieces 11 and 12 by delicate membranes and having the same gas on both sides of the detector 21 or having a slow flow of flush gas constantly moving in the direction away from parts 11 and 12 toward the detector, the flow being slow enough to faithfully transmit pressure changes from the zone of the pieces 11 and 12 to the detector.

Reference gas, ambient air or may be used as the flush gas.

Iclaim:

1. In an oxygen analyzer for quick response indication wherein there is a nearly closed ferromagnetic circuit magnet having a magnet gap chamber having small conduits for the entrance into the chamber of analysis gas and gas of known magnetic susceptibility and said gases leave the chamber by a common exit, and wherein said magnet is periodically excited and means are provided for measuring differential gas pressures in the respective conduits, the improvement comprising said magnet having substantially opposite pole end portions of substantially uniform cross section and parallel end faces, means for forming said chamber and confining said chamber to an area at least within said cross section so that the chamber will be substantially within a homogenous magnetic field when the magnet is excited, a nonhomogenous field being inherently produced in the zone around the homogenous field, and valves for regulating the flow of the gases through the conduits, exit and chamber so that when the gases in the conduits have sufficient linear speed mutual diffusion and intermixing of the analysis and reference gas can take place only in the homogeneous zone and the exit so that always in the inhomogeneous zone of the field each conduit is filled exclusively with the respective gas, said means for forming including a strong plate of nonferromagnetic material interposed essentially gastight between said faces to reduce magnetomechanical deformation of the chamber, the plate being provided with a longitudinal cutout in the mid portion thereof to form the chamber and within said cross section, the conduits opening into the cutout about at its respective ends and the common exit being about in the middle of the cutout.

2. ln an analyzer as claimed in claim 1 and plates of high magnetic permeability on each side of the nonferromagnetic plate for reducing magnetomechanical and magnetostrictive force on the chamber and engaging on the end faces of the magnet as an insert fitted thereon.

3. In an analyzer as claimed in claim 2, said plates being provided with passageways as parts of the conduits and exit means.

gas of special composition 

2. In an analyzer as claimed in claim 1 and plates of high magnetic permeability on each side of the nonferromagnetic plate for reducing magnetomechanical and magnetostrictive force on the chamber and engaging on the end faces of the magnet as an insert fitted thereon.
 3. In an analyzer as claimed in claim 2, said plates being provided with passageways as parts of the conduits and exit means. 